Gas discharge lamps produce light through the ignition and stabilization of an electric arc. An electric arc is the electrical breakdown of a gas which produces an ongoing plasma discharge. This electrical breakdown is achieved by applying an electrical field across the lamp. Once the breakdown occurs and the plasma is formed, electrons will flow through the plasma and its composite molecules from one terminal of the lamp to another. When the electrons collide with the composite molecules of the plasma they excite electrons on those molecules to a higher state. These electrons then emit a substantially equivalent amount of energy in the form of visible light as they descend back to their quiescent states.
Although electric arcs are extremely efficient, formation of an arc requires a significant pulse of energy. In gas discharge lamps, an arc is initially struck by applying an ignition pulse to the lamp. This process is referred to as “igniting” or “striking” the lamp. The ignition pulse will usually be a large voltage pulse applied across the terminals of the lamp. The voltage pulse needs to be high enough to exceed the electric breakdown of the chemicals within the lamp. The voltage required to start the lamp is a function of many different variables including the chemicals within the lamp, the temperature of those chemicals, and the general architecture of the lamp.
The relationship between temperature and the required magnitude of an ignition pulse is the root cause of a significant drawback inherent in gas discharge lighting. A lamp that produces light also produces heat which raises the temperature of the chemicals within the lamp. When the chemicals within a gas discharge lamp are heated, more energy is required to ignite the lamp. These aspects of gas discharge lamps combine to form what is called the hot reignition—or hot restrike—problem. If a lamp has been running for an appreciable amount of time, and is then shut-off, it will be extremely difficult to turn the lamp back on again. Often times the pulse of energy that a system was designed to apply to the lamp under usual conditions will not be sufficient to reignite the lamp. In such cases, the hot reignition problem will lead to a situation where light will not be available from the lamp until enough time has passed for the lamp to sufficiently cool. In applications where continuous and responsive lighting is critical, this is an unacceptable condition.
The hot reignition problem has been recognized in the field of gas discharge lamps since its inception. Early approaches to this problem included leaving the lights on permanently and covering the lights with movable metal shutters to block the light when it wasn't needed. This approach increases the light source's responsiveness, but is also clearly power inefficient. Another family of early approaches involved applying a much larger or specially shaped ignition pulse to the lamp so that enough energy was applied to start the lamp even if the chemicals inside were still in an excited state. Although this approach decreases the start-up time of the lamps as compared to the approach of waiting for the lamp to cool, this approach could cause serious damage to the lamps because of the high energy levels required for hot-reignition pulses.
Lamp ignition stresses are a serious cause of lamp life degradation. As such, it is important to prevent a lighting system from conducting ignitions that fail to ignite the lamp and thereby needlessly tax the lamp's components. Needless strikes can also waste power as in the situation of a “cycling” lamp that is continuously reignited and extinguished because it has degraded. From the perspective of the hot reingition problem, failed reignition attempts are also harmful because failed attempts apply energy to the lamp and increase the lamp's temperature thereby extending the time that it will take for the lamp to cool. Inventions that seek to limit the number of failed ignitions relate to the problem of hot reignition because they all seek to eliminate the condition where a fruitless ignition pulse is delivered to a lamp that only serves to wear out the lamp components and waste energy.
There is prior art dealing with the problem of preventing unnecessary ignition attempts by limiting the number of reignition attempts after a certain amount of time, or after a certain amount of attempts. For example, there are approaches wherein a series of reignition pulses are turned off after a certain time period has elapsed. Likewise, a circuit may automatically detect if failed reignition pulses have generated a certain amount of heat, at which point the ignition circuit is disabled for a period of time. These approaches are particularly suited for a situation where specialized high-power reignition pulses are being applied because such pulses are even more likely to damage a lamp than regular strength ignition pulses. These approaches share the debilitating characteristic of taking action after the ignition pulses have failed. The main problem with approaches that cease attempting to ignite a lamp after an initial series of fruitless strikes is that the initial strikes still wear down the lamp, waste energy, reheat the lamp, and increase the overall time that must pass before the lamp is sufficiently cooled for reignition.
As compared to needless striking due to cycling or malfunctioning sensors, needless striking due to the hot reignition problem is somewhat more manageable. This is because a lamp's temperature can be measured directly, or estimated based on knowledge of how long it has been since the lamp went out. Therefore, many approaches in the prior art are focused on not allowing a reignition attempt for a certain period of time after the lamp has been shut off. For example, this approach may be used to protect delicate gas discharge bulbs in LCD screens from hot reignition attempts. In this example, a circuit monitors when a power off signal has been received, keeps track of how much time has passed since that signal was received, and prevents the reignition of the lamp until a certain amount of time has passed by not allowing a power on signal to trigger a reignition. Similarly, one may use a system for generating and supplying power wherein some of the devices being supplied may be gas discharge lamps. In this example, one may wait a certain amount of time after a brown-out to allow the lamps to cool before returning power to the lamps. In a similar approach, the operating conditions of the lamp are monitored directly to determine the state of the lamp, and a certain amount of time passes after a fault condition is detected before a reignition signal is sent. A control circuit monitors the time since the failure condition was detected, and sends the control signal to the lamp when it is time to reignite.
The approaches discussed in the previous paragraph all share the common drawback of requiring a separate circuit or system to keep track of how much time has passed since the lamp has gone out. In addition, many hot reignition situations occur because there is a momentary blip in the power supplied to the lamp. In these situations, separate circuitry powered by the same power supply as the lamp may be unable to function properly and assure that the system retains memory of when the lamp went out.